1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a computer readable recording medium storing a program, each performing screen processing.
2. Description of Related Art
Forgery of a negotiable instrument, a passport and the like has been frequently committed as the improvement of the digital image processing and printing techniques in a color copier, a printer and the like in recent years. Accordingly, development of the techniques for preventing forgery has been performed vigorously, and the developed techniques are used.
For example, there has been disclosed a technique capable of genuineness determination of original or duplicate based on intervals of each dot constituting dotted lines which are drawn in an image together with curved lines, in which the original image is drawn to have a structure of arranging the curved lines and the dotted lines arranged interleavingly (see, for example, JP 2005-28645A). Moreover, there has been also disclosed a technique capable of the genuineness determination based on the shape of halftone dots of a printed image by using a particular screen cell. In this technique, the shape of a screen cell in dither method has an elaborate design when half tone is reproduced, (see, for example, JP 11-268229A).
Moreover, as one of the techniques for preventing forgery, a sketch-like painting technique using curved lines (hereinafter referred to as a pictorial technique) can be cited. The technique is a printing technique represented by a copperplate print technique used in, for example, a portrait printed on a banknote, or the like. In this technique, grooves are directly formed on a copperplate with a graver or the like to create a plate, and an ink is filled in the grooves on the plate to be transferred on printing paper. Thus a printed matter can be obtained.
Light and shade on a printed matter is adjusted based on widths and the depths of the grooves, and the grooves are formed by hand. Consequently, an image formed on the printed matter is naturally expressed by curved lines.
Drawing a geometric pattern, a person and the like by such an expression using curved lines enables to acquire a forgery prevention effect. The reason is as follows.
As a first reason, it can be cited that it is difficult to express a curved line is by the present half tone reproduction technique in an image processing apparatus.
In an image processing apparatus, a dither method, error diffusion method and the like are used as the half tone reproduction technique. The former method is to express light and shade (half tone) by using a dot pattern repeated in a comparatively short uniform period and by controlling the sizes of the dots. The latter method is to express light and shade by controlling density of the dots (the number of dots) per a unit area, although the sizes of the dots themselves are uniform. In these methods, it is impossible to express light and shade at least by using gentle curved lines like in a copperplate print. As for the dither method, the reason of the impossibility is that the pattern period is limited to about several pixels owing to the cost thereof and it is difficult to obtain the pattern period sufficient for drawing curved lines. As for the error diffusion method, the reason of the impossibility is that light and shade is basically expressed only by density of dispersed dot having no periodicity.
That is, a new technique is required to realize the half tone reproduction using gentle curved lines, and such a technique has not been established. Moreover, even if such a technique is established, because a copperplate print used for a specific negotiable instrument is hand-carved, its lines are complicated and it is difficult to reproduce quite the same streaks even if a specific function is used.
As a second reason, a problem of reading resolution of a scanner can be cited. For committing forgery, an image of a negotiable instrument or the like is read with a scanner, and the read image is printed. When a pattern or the like drawn with curved lines is read with a scanner, moiré sometimes occurs. The reason of the moiré is that a curved line pattern includes various spacial frequencies. Because image quality is deteriorated when moiré occurs, whether the image is an original or not can be easily discriminated.
As a third reason, the deterioration of image quality caused by erroneous discrimination of image species in an image processing apparatus can be cited. A general image processing apparatus discriminates an image species such as a character, photographic image or the like so as to perform different image processing to each image species. However, the accuracy thereof is not perfect, and erroneous discrimination is sometimes preformed. As a discrimination method, there is a method of cutting out a local region to discriminate whether the cut local region is a character or an image other than the character by pattern matching. Because the local region to be referred to is only a region corresponding to several pixels, erroneous discrimination can be performed. Because the deterioration of image quality occurs in the part where such erroneous discrimination is performed, the discrimination of whether the image is the original or not can be made based on the deteriorated part of image quality.
Moreover, one of the methods of discriminating an image portion other than characters is to detect the halftone dot structure of a printed matter. Specifically, the method is the method of detecting the periodicity of light and shade (or screen ruling) by pattern matching. However, the range in which the periodicity can be detected is limited. If the screen ruling is within this range, it is possible to discriminate the halftone dot image portion of a printed matter. Accordingly, it is possible to keep accuracy of the discrimination by adjusting the detectable period by the adjustment of a parameter or the like according to each printed matter. This is because it is rare to use the halftone dot structures having the mutually different screen ruling on one same page due to a cost problem, although the screen ruling used in a printed matter is various according to the uses thereof.
However, if such a discrimination technique is applied to an image having a halftone dot structure in which the period of halftone dots gently varies over a wide range, then erroneous discrimination is caused due to the gentle variation. Because the erroneously discriminated portion of a gentle variation is very conspicuous, it is possible to easily judge whether a printed matter is a copy or not by visual observation based on the portion.
As described above, the painting of an image by the pictorial technique with curved lines has a forgery prevention effect.
However, as described above, there are no techniques of reproducing a gentle curved line pattern into half tones by image processing. Although the technique described in the JP 2005-28645A can draw a curved line, but the technique is not for reproducing half tones by the image processing of the dither method or the like.
Moreover, a matter having a high added value such as a negotiable instrument is worth providing a high-grade forgery preventing measure such as a hologram, IC chip or the like and spending a large cost. However, as for a printed matter such as a mere voucher, which has a low added value and is wanted to be printed with a general copier or a printer simply, and is also wanted to be prevented from forgery. Thus, forgery prevention at low cost is required.